1. Field of the Invention
The present invention relates to a system and method for manufacturing a thin-film transistor of bottom-gate structure, having a gate electrode formed between the substrate and a polysilicon film. The invention also relates to a method of evaluating a polysilicon film and an apparatus for inspecting a polysilicon film.
2. Related Art
In recent years, thin-film transistors with a channel layer that is made of polysilicon have been put to practical use. Thin-film transistors having a channel layer made of polysilicon exhibits very high electric-field mobility. When used in the drive circuit of, for example, a liquid crystal display, these thin-film transistors will enable the display of images of high color quality, operate at high speed, and will render the display smaller in size.
In recent years, the so-called xe2x80x9clow-temperature poly-crystallization processxe2x80x9d has been developed. In this process, an excimer laser applies heat to amorphous silicon, forming a polysilicon film. The process may be applied to a method of manufacturing thin-film transistors, thereby to suppress thermal damage to the glass substrate and utilize a large, inexpensive heat-resistant glass substrate.
Excimer lasers, which may be used in the low-temperature polycrystallization process, output but an unstable power. The grain size of the polysilicon film they form inevitably varies so much. The polysilicon film may not always have a desired grain size. The silicon crystals may be too small, resulting in so-called xe2x80x9clinear error.xe2x80x9d Conversely, they may be too large, resulting in so-called xe2x80x9cwrite error.xe2x80x9d
In most processes of annealing amorphous silicon by an excimer laser, the data representing the energy applied to the amorphous silicon is fed back to the escimer laser. Based on the information, the energy density of the excimer laser beam is changed to an optimal one.
The visual or sensual inspection is only one method available for evaluating the resultant polysilicon film. In the sensual method, the surface of the film is photographed by the use of a spectral ellipsometer, a scanning electron microscope, or the like, and the photograph is examined to determine the crystal condition of the film. In other words, the crystal condition of the film cannot be determined by any non-contact, objective inspections.
The present invention has been made in consideration of the foregoing. An object of the invention is to provide a system and method for manufacturing thin-film transistors having a polycrystalline film formed by using an laser annealing apparatus that emits a laser beam having energy density optimized based on the crystal condition of the film, which has been evaluated by an objective, non-contact, automatic inspection at high accuracy. Another object of the invention is to provide a method of evaluating such a polysilicon film. A further object is to provide an apparatus for inspecting such a polysilicon film.
A system for manufacturing a thin-film transistor, according to the present invention comprises: a film-forming apparatus for forming a metal pattern on a substrate and forming amorphous silicon film on the substrate, covering the metal pattern; a laser annealing apparatus for annealing the amorphous silicon film, converting the same to a polysilicon film that serves as a channel layer; observation means for observing a surface spatial structure of the polysilicon film; and a polysilicon-inspecting apparatus for evaluating the surface spatial structure observed by the observation means, thereby to determine the condition of the polysilicon film. The polysilicon-inspecting apparatus evaluates, in numerical values, the surface spatial structure of a part of the polysilicon film, which lie above the metal pattern, and the surface spatial structure of the other parts of the polysilicon film. The polysilicon-inspecting apparatus calculates a difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film. It determines the condition of the polysilicon film from the difference calculated. The laser annealing apparatus uses the difference as a control parameter for controlling the energy density of the laser beam, and the film-forming apparatus uses the difference as a control parameter for controlling the thickness of the polysilicon film.
In the system, the surface spatial structure of that part of the polysilicon film, which lies above the metal pattern, is evaluated in a numerical value, and the surface spatial structure of the other parts of the polysilicon film is evaluated in a numerical value, too. Then, a difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film is calculated. The energy density of the laser beam or the thickness of the polysilicon film is set by using the difference as a control parameter.
In a method of manufacturing a thin-film transistor, according to this invention, a laser annealing apparatus anneals an amorphous silicon film, thus converting the film to a polysilicon film that serves as a channel layer. The method comprises: forming a metal pattern on a substrate; forming an amorphous silicon film on the substrate, thereby covering the metal pattern; performing laser annealing on a plurality of parts of the amorphous silicon film formed on one or more substrates, at different energy densities of a laser beam, thereby forming a polysilicon film; evaluating, in numerical value, the surface spatial structure of that part of the polysilicon film, which lies above the metal pattern; evaluating, in numerical value, the surface spatial structure of the other parts of the polysilicon film; calculating a difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film; and controlling the energy density of the laser beam or the thickness of the polysilicon film, by using the difference as a control parameter for controlling the energy density and the thickness.
In the method of manufacturing a thin-film transistor, the surface spatial structure of that part of the polysilicon film formed by means of the laser annealing apparatus, which lies above the metal pattern, is evaluated in a numerical value, and the surface spatial structure of the other parts of the polysilicon film is evaluated in a numerical value, too. A difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film is calculated. The energy density of the laser beam or the thickness of the polysilicon film is set by using the difference as a control parameter.
According to the invention there is provided a method of evaluating a polysilicon film serving as a channel layer and provided in a thin-film transistor that comprises a substrate, a metal pattern provided on the substrate and a channel layer, said polysilicon film having been formed by forming an amorphous silicon film on the substrate and performing laser annealing on the amorphous silicon film. The method comprises: evaluating a surface spatial structure of a part of the polysilicon film, which lines above the metal pattern, in a numerical value; evaluating surface spatial structure of other parts of the polysilicon film, in a numerical value; and calculating a difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film, and evaluating the polysilicon film based on the difference calculated.
In the method of evaluating a polysilicon film, the surface spatial structure of the polysilicon film formed by the laser annealing is evaluated in a numerical value. The condition of the polysilicon film is evaluated based on the difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film is calculated.
According to the invention there is provided an apparatus for inspecting a polysilicon film serving as a channel layer and provided in a thin-film transistor that comprises a substrate, a metal pattern provided on the substrate and a channel layer, said polysilicon film having been formed by forming an amorphous silicon film on the substrate and performing laser annealing on the amorphous silicon film. The apparatus for inspecting a polysilicon film comprises: observation means for observing a surface spatial structure of the polysilicon film; and inspecting means for inspecting the surface spatial structure observed by the observation means, thereby to determine the condition of the polysilicon film. The inspecting means evaluates, in numerical values, the surface spatial structure of a part of the polysilicon film, which lie above the metal pattern, and the surface spatial structure of the other parts of the polysilicon film, calculates a difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film, and determines the condition of the polysilicon film from the difference calculated.
In the apparatus for inspecting a polysilicon film, the surface spatial structure of the polysilicon film formed by the laser annealing is evaluated in a numerical value. The condition of the polysilicon film is evaluated based on the difference between the numerical value of the part of the polysilicon film, which lies above the metal pattern, and the numerical value of the other parts of polysilicon film is calculated.
In the system and method for manufacturing thin-film transistors, both according to the present invention, the surface spatial structure of a polysilicon film formed by a laser annealing apparatus is evaluated in a numerical value. A difference between a part of the polysilicon film, which lies above a metal pattern, and the other parts of the polysilicon film is calculated in terms of the numerical values. The difference calculated is used as a control parameter, adjusting the energy density of the laser beam emitted by the laser annealing apparatus or setting the thickness of the polysilicon film.
With the system and method it is therefore possible to set the intensity of the laser beam emitted from the laser annealing apparatus, at an optimal value. This can enhance the yield of the thin-film transistor.
In the method of evaluating a polysilicon film, according to this invention, the surface spatial structure of a polysilicon film formed by a laser annealing apparatus is evaluated in a numerical value. A difference between a part of the polysilicon film, which lies above a metal pattern, and the other parts of the polysilicon film is calculated in terms of the numerical value. The condition of the polysilicon film is evaluated from the difference thus calculated.
With this method it is easy to evaluate the polysilicon film in a non-destructive manner. In addition, it is possible to evaluate the film in a numerical value, unlike in a visual or sensual inspection. Hence, the polysilicon film can be automatically evaluated. Thus, an objective inspection can be achieved at high accuracy.
In the apparatus for inspecting a polysilicon film, according to the invention, the surface spatial structure of a polysilicon film formed by a laser annealing apparatus is evaluated in a numerical value. A difference between a part of the polysilicon film, which lies above a metal pattern, and the other parts of the polysilicon film is calculated in terms of the numerical value. The condition of the polysilicon film is evaluated from the difference thus calculated.
The inspection apparatus can easily evaluate the polysilicon film in a non-destructive manner. Additionally, it can evaluate the film in a numerical value, unlike in a visual or sensual inspection. Therefore, it can evaluate the polysilicon film automatically. Thus, the apparatus can perform an objective, high-accuracy inspection.